| Virgo Stellar Stream | |
|---|---|
 Stellar streams (artist's impression) | |
| Observation data (J2000epoch) | |
| Constellation | Virgo |
| Right ascension | ? |
| Declination | ? |
| Distance | 30kly |
| Apparent magnitude (V) | ? |
| Absolute magnitude (V) | ? |
| Characteristics | |
| Type | remains of a dSph |
| Apparent size (V) | 30° × 10° |
| Other designations | |
| none | |
TheVirgo Stellar Stream, also known asVirgo Overdensity, is the proposed name for astellar stream in theconstellation ofVirgo which was discovered in 2005.[1][2] The stream is thought to be the remains of adwarf spheroidal galaxy that is in the process of merging with theMilky Way. It is the largestgalaxy visible from theEarth, in terms of thearea of thenight sky covered.
The stream was discovered fromphotometric data from theSloan Digital Sky Survey, which was used to create athree-dimensional map of the Milky Way, using thecolors andbrightness of certaincharacteristictypes of stars to estimate their distance (a method known as "photometric parallax").[3] The first suggestion of a new galaxy in Virgo was made in 2001 from data obtained as part of theQUEST survey, which used the one-metreSchmidt telescope at theLlano del Hato National Astronomical Observatory inVenezuela to search forRR Lyraevariable stars. Five were found in a clump with aright ascension near 12.4 hours, and the astronomers speculated that this clump was part of a small galaxy being "cannibalised" by the Milky Way.[4]
The stream covers over one hundredsquare degrees and possibly as much as one thousand square degrees (approximately five percent of thehemisphere visible at any one time, or five thousand times the area of thefull moon). Despite its proximity to theSolar System and thesolid angle that it consequently covers, the stream contains only a few hundred thousand stars. The lowsurface brightness of the galaxy (possibly as low as 32.5 mag/arcmin2) may have militated against its detection in surveys before SDSS. The number of stars in the stream is not greatly in excess of astar cluster, and it has been described by a member of the team that discovered it as "a rather pathetic galaxy" in comparison to the Milky Way.[5] Many of the stars have been known for centuries and thought of as normal Milky Way stars, although they have a lowermetallicity than normalPopulation I stars in the Milky Way.
The stream lies within the Milky Way, approximately 10 kiloparsecs (30,000 light-years) from theSun, and extending over a region of space at least 10 kpc across in three dimensions. It is close on the plane of the sky to theSagittarius Dwarf Elliptical Galaxy, which was found in 1994 through a similar photometric analysis of a star survey.[6] The Sagittarius Dwarf is another small galaxy which is also in the process of merging with the Milky Way; however, it is approximately 4 times further away than the stream, so the two are unlikely to be physically related, although it is possible that the Virgo Stellar Stream is a remnant left behind by the disruption of the Sagittarius Dwarf as it had orbited around the Milky Way.[7] The Virgo Stellar Stream also resembles theMonoceros Ring, found in 2002,[8] which has similarly been attributed to theCanis Major Dwarf Galaxy merging with the Milky Way.
